The dining room floor in my 175 year old house sags toward the center of the house. I have tried to jack up the 2 or 3 joists that are responsible, but they don't want to move much. There is no rot or structural damage, just settling I guess. I have the joists well-supported now by basement posts. The floor is the original hardwood, which will look great when it is sanded and refinished -- and leveled! But how? I was thinking I could cut the subfloor loose from the joists (run a reciprocating saw between the joist and subfloor to cut the nails) and then push up the floor from the basement off the joists until it's level and nail sister joists onto the joists to support the subfloor. Crazy?

Hi Jack,
The floor sags deepest at the joist that supports a load bearing wall running along it in the center of the house. The upstairs floor above it sags in the same way. I have the joists temporarily supported by several 8' pole floor jacks that I will replace with poles mounted on footers.
Bill

Although jack posts work well for supporting they are not that good at lifting. You can use a hydraulic jack set on a solid base to lift a little then crank up the jack posts or you can rent screw jack made for this purpose. With the proper jack you can lift the entire house. I would try this before cutting the sub-floor loose as you suggested.

Also keep in mind that the sag didn't occur overnight, it took many years for the materials to deflect to their current state. Don't try to remove the entire sag in one jacking session, nor in one spot. A series of jacks along the area to be lifted OR lift slightly, support, move down and lift again, repeat until you get to the end of the sagging section. I would say that you don't want to lift more than 1/8" to 1/4" at a time, wait several days and do it again.

Thanks guys. Over several months I have used a hydraulic jack supported on a solid base to lift one end a small amount at a time, then I raise the pole jacks to support, then move down, as you suggested. But it got to the point that it was not seeming to want to move any more. That joist supports a center interior wall that supports the span of floor above it. I got worried about trying to push all that up and then have to support it.